Python learning notes -- Using the Python Interpreter

Using the Python Interpreter

2.1. Invoking the Interpreter notebook ¶

The Python interpreter is usually installed as/usr/local/bin/python on those machines where it is available; putting/usr/local/bin in your Unix shell's search path makes it possible to start it by typing the command

Python

To the shell. since the choice of the directory where the interpreter lives is an installation option, other places are possible; check with your local Python guru or system administrator. (E. g ., /usr/local/python is a popular alternative location .)

On Windows machines, the Python installation is usually placed in C: \ Python27, though you can change this when you're running the installer. to add this directory to your path, you can type the following command into the command prompt in a DOS box:

Set path = % path %; C: \ python27

Typing an end-of-file character (Control-D on Unix, Control-Z on Windows) at the primary prompt causes the interpreter to exit with a zero exit status. if that doesn't work, you can exit the interpreter by typing the following command: quit ().

The interpreter's line-editing features usually aren't very sophisticated. on Unix, whoever installed the interpreter may have enabled support for the GNU readline library, which adds more elaborate interactive editing and history features. perhaps the quickest
Check to see whether command line editing is supported is typing Control-P to the first Python prompt you get. If it beeps, you have command line editing; see Appendix
Interactive Input Editing and History SubstitutionFor an introduction to the keys. If nothing appears to happen, or if ^ P is echoed, command line editing isn't available;
You'll only be able to use backspace to remove characters from the current line.

The interpreter operates somewhat like the Unix shell: when called with standard input connected to a tty device, it reads and executes commands interactively; when called with a file name argument or with a file as standard input, it reads and executes
AScriptFrom that file.

A second way of starting the interpreter is python-c command [arg]..., which executes the statement (s) in
Command, Analogous to the shell's
-COption. Since Python statements often contain spaces or other characters that are special to the shell, it is usually advised to quote
CommandIn its entirety with single quotes.

Some Python modules are also useful as scripts. These can be invoked using python-m module [arg]..., which executes the source file
ModuleAs if you had spelled out its full name on the command line.

When a script file is used, it is sometimes useful to be able to run the script and enter interactive mode afterwards. This can be done by passing

-IBefore the script. (This does not work if the script is read from standard input, for the same reason as explained in the previous paragraph .)

2.1.1. Argument Passing Enabled ¶

When known to the interpreter, the script name and additional arguments thereafter are turned into a list of strings and assigned to the argv variable in the sys module. you can access this list by executing import sys. the length of the list is at least
One; when no script and no arguments are given, sys. argv [0] is an empty string. when the script name is given as '-' (meaning standard input), sys. argv [0] is set '-'. when

-C CommandIs used, sys. argv [0] is set to '-C'. When
-M ModuleIs used, sys. argv [0] is set to the full name of the located module. Options found after

-C CommandOr
-M ModuleAre not consumed by the Python interpreter's option processing but left in sys. argv for the command or module to handle.

2.1.2. Interactive Mode Enabled ¶

When commands are read from a tty, the interpreter is said to be inInteractive mode. In this mode it prompts for the next command with
Primary prompt, Usually three greater-than signs (>>>); for continuation lines it prompts with
Secondary prompt, By default three dots (...). The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt:

Python

Python 2.7 (#1, Feb 28 2010, 00:02:06)

Type "help", "copyright", "credits" or "license" for more information.

>>>

Continuation lines are needed when entering a multi-line construct. As an example, take a look at this
IfStatement:

>>>The_world_is_flat = 1

>>> IfThe_world_is_flat:

... Print"Be careful not to fall off! "

...

Be careful not to fall off!

2.2. The Interpreter and Its Environment timeout ¶

2.2.1. Error Handling handle ¶

When an error occurs, the interpreter prints an error message and a stack trace. in interactive mode, it then returns to the primary prompt; when input came from a file, it exits with a nonzero exit status after printing the stack trace. (Exceptions handled
ByExceptClause in
TryStatement are not errors in this context.) Some errors are unconditionally fatal and cause an exit with a nonzero exit; this applies to internal inconsistencies and
Some cases of running out of memory. All error messages are written to the standard error stream; normal output from executed commands is written to standard output.

Typing the interrupt character (usually Control-C or DEL) to the primary or secondary prompt cancels the input and returns to the primary prompt.
[1] Typing an interrupt while a command is executing raises

KeyboardInterruptException, which may be handled by
TryStatement.

2.2.2. Executable Python Scripts ¶

On BSD 'ish Unix systems, Python scripts can be made directly executable, like shell scripts, by putting the line

#! /Usr/bin/env python

(Assuming that the interpreter is on the user'sPATH) At the beginning of the script and giving the file an executable mode. #! Must be the first two characters of the file. On some platforms, this first line must end with a Unix-style
Line ending ('\ n'), not a Windows (' \ r \ n') line ending. note that the hash, or pound, character, '#', is used to start a comment in Python.

The script can be given an executable mode, or permission, usingChmodCommand:

$ Chmod + x myscript. py

On Windows systems, there is no notion of an "executable mode ". the Python installer automatically associates. py files with python.exe so that a double-click on a Python file will run it as a script. the extension can also be. pyw, in that case, the console
Window that normally appears is suppressed.

2.2.3. Source Code Encoding ¶

It is possible to use encodings different than ASCII in Python source files. The best way to do it is to put one more special comment line right after #! Line to define the source file encoding:

#-*-Coding: encoding -*-

With that declaration, all characters in the source file will be treated as having the encoding
Encoding, And it will be possible to directly write Unicode string literals in the selected encoding. The list of possible encodings can be found in the Python Library Reference, in the section on

Codecs.

For example, to write Unicode literals including the Euro currency symbol, the ISO-8859-15 encoding can be used, with the Euro symbol having the ordinal value 164. this script will print the value 8364 (the Unicode codepoint corresponding to the Euro symbol)
And then exit:

#-*-Coding: iso-8859-15 -*-

 

Currency = u "€"

PrintOrd (currency)

If your editor supports saving files as UTF-8 with a UTF-8Byte order mark(Aka BOM), you can use that instead of an encoding declaration. IDLE supports this capability if Options/General/Default Source Encoding/UTF-8 is set. Notice that this signature
Is not understood in older Python releases (2.2 and earlier), and also not understood by the operating system for script files #! Lines (only used on Unix systems ).

By using UTF-8 (either through the signature or an encoding declaration), characters of most editions in the world can be used simultaneously in string literals and comments. using non-ASCII characters in identifiers is not supported. to display all these
Characters properly, your editor must recognize that the file is UTF-8, and it must use a font that supports all the characters in the file.

2.2.4. The Interactive Startup File contains ¶

When you use Python interactively, it is frequently handy to have some standard commands executed every time the interpreter is started. You can do this by setting an environment variable named
PYTHONSTARTUPTo the name of a file containing your start-up commands. This is similar to the. profile feature of the Unix shells.

This file is only read in interactive sessions, not when Python reads commands from a script, and not when/dev/tty is given as the explicit source of commands (which otherwise behaves like an interactive session ). it is executed in the same namespace where
Interactive commands are executed, so that objects that it defines or imports can be used without qualification in the interactive session. You can also change the prompts sys. ps1 and sys. ps2 in this file.

If you want to read an additional start-up file from the current directory, you can program this in the global start-up file using code like if OS. path. isfile ('. pythonrc. py '): execfile ('. pythonrc. py '). if you want to use the startup file in a script, you
Must do this explicitly in the script:

Import OS

Filename = OS. environ. get ('pythonstartup ')

IfFilenameAndOS. path. isfile (filename ):

Execfile (filename)

Footnotes

[1]	A problem with the GNU Readline package may prevent this.

 

 

 

 

 

 

 

 

Use the Python Interpreter

2.1 call Interpreter

The Python interpreter is usually located in the path/usr/local/bin/python, and can be opened by entering python in the UNIX command line. You can also install Python in other paths. In Windows, Python is usually located in the C: \ Python27 path.

The interpreter editing function is generally not powerful enough, while the GNU readline library can provide programmers with more powerful interactive programming tools.

To confirm whether the command line editing is supported, click Ctrl + P in your first Python program. If the buzzer rings, the command line editing mode is supported.

The Python interpreter works like a Unix script program: if the standard input and output are called, the program will input and output data from the command line; if a variable named after a file name is called or a file is dropped, the program reads and runs the script.

The second method to open the Python interpreter is to input the following in the command line:

Python-c command [arg]...

It runs the script through the command line. Because Python programs often contain spaces or other characters that are very special to the interpreter, we strongly recommend that you use the single quotation mark reference command.

Some Python modules can also be used as very useful scripts. They can be called using the python-m module [arg]... statement.

Sometimes it is very useful to call a script file through interactive running. It can be implemented by passing the-I parameter. (However, if the program does not use standard input and output, the interactive method cannot be implemented .)

 

2.1.1 parameter transfer

When the command is detected by the interpreter, the Script Name and parameters are also converted into a string list and assigned to the argv variable in the sys module, you can run import sys to view the list with at least 1 length. If there is no script or parameter, sys. argv [0] is an empty string.

 

2.1.2 Interactive Mode

When the command is executed in standard input/output mode, we say it runs in interactive mode.

Python

Python 2.7 (#1, Feb 28 2010, 00:02:06)

Type "help", "copyright", "credits" or "license" for more information.

>>>

 

>>>The_world_is_flat = 1

>>> IfThe_world_is_flat:

... Print"Be careful not to fall off! "

...

Be careful not to fall off!

 

2.2 interpreter and Working Environment

2.2.1 error handling

When an error occurs, the interpreter prints an error message and its stack call. In interactive mode, it returns the primary prompt. When the input is from a file, the program prints the stack call status and returns a non-zero value. (Exception handling is not considered as error handling here .)

All error messages are written to the standard output stream.

When an interrupt is introduced in the command lineKeyboardInterruptException, which must be handled by try.

2.2.2 executable Python script

In bsd unix, Python scripts can be run directly. Write at the beginning of the source file

#! /Usr/bin/env python

This file can be directly run.

Use chmod to make the script run.

$ Chmod + x myscript. py

2.2.3 code writing

You can use different encoding methods than ASCII in Python source code. The best way is in #! Write comments.

#-*-Coding: encoding -*-

By using this method, all characters in the source file are processed as the encoding method. You can also use the UNICODE encoding method directly. All supported encoding methods of Python can be found in the Python Library Reference.

2.2.4 run the program in interactive mode

When you use Python in interactive mode, some commands are always called when the interpreter runs. You can implement this by defining the environment variable PYTHONSTARTUP. It is similar to. profile in UNIX.

This type of file is read only in interactive mode, but not when Python reads the script file. They are executed under the same namespace, so objects can be used in interactive mode.

If you want to read an additional Startup file in the current directory, you can define it in the global Startup File as follows:

Import OS

Filename = OS. environ. get ('pythonstartup ')

IfFilenameAndOS. path. isfile (filename ):

Execfile (filename)

 

[Appendix]

GNU readline is a software package created and maintained by the GNU project. It provides the row editing function. For example, in a readline-authorized application, press C-B (CTRL-B) to move the pointer back to a position, but C-f moves the Pointer Forward to a position; c-r searches in historical records; these key combinations (which are default, although combinations of vi's are optional) come from the earliest and most popular GNU project, text Editor Emacs. Readline supports multiple advanced features, including a kill ring (a more flexible version of the copy/paste clipboard) and tab completion
. As a cross-platform library, readline allows many applications on multiple systems to display row editing behaviors. It is a free package distributed under the GNU General Public License (GPL.